Field of the Invention
The invention concerns a projection objective of a microlithographic projection exposure apparatus designed for EUV and a method of optically adjusting a projection objective.
Background
Microlithography is used for the production of microstructured components such as for example integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination system and a projection objective. In that case the image of a mask (=reticle) illuminated via the illumination system is projected via the projection objective onto a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective to transfer the mask structure onto the light-sensitive coating on the substrate.
Mirrors are used as optical components for the imaging process in projection objectives designed for the EUV range, that is to say at wavelengths of for example about 13 nm or about 7 nm, due to the lack of availability of suitable translucent refractive materials.
Typical projection objectives designed for EUV as are known for example from U.S. Pat. No. 7,538,856 B2 can for example have an image-side numerical aperture (NA) in the range from 0.2 to 0.3 and reproduce an object field (for example which is annular) in the image plane or wafer plane.
In the case of approaches for increasing the image-side numerical aperture (NA), a problem which arises in practice is that limits are set in many respects on an increase in the size of the mirror surfaces, that is linked to that increase in the numerical aperture; on the one hand, with increasing dimensions of the mirrors, it becomes increasingly difficult to reduce in particular long-wave surface errors to values below the required limit values, in which respect the larger mirror surfaces require inter alia greater aspheres. In addition increasing dimensions for the mirrors require larger processing machines for manufacture, and stricter requirements are placed on the machining tools used (such as for example grinding, lapping and polishing machines, interferometers and cleaning and coating installations). In addition, the manufacture of larger mirrors makes it necessary to use heavier mirror base bodies which, as from a certain limit, can scarcely still be mounted in position or flex beyond an acceptable degree due to the force of gravity.
A further problem involved with the increase in mirror dimensions results from shadowing of regions of the illumination beam path. In that respect it is admittedly possible to use systems with central obscuration but therein the above-described problems still arise. It is known inter alia from WO 2008/020965 A2 to produce a collector mirror of the EUV light source by a plurality of discrete substrate being directed towards a common focal point and then each coated with a multiple layer which is reflective for EUV. To orient the collector mirror which for example is in the form of an ellipsoidal mirror one or more actuators are used for orientation of at least one of the substrate relative to a carrier structure, such orientation process being effected in dependence on a measurement of the light which is deflected from a first focal point to a second focal point of the ellipsoidal mirror.